With the depth 2 175's should do fine. You could use 250's but it would be more heat to deal with and no real gain. You have to remember the more wattage means more heat. This is just my oppinion but I would opt for 2 175's. In a tank of this size going overkill on lighting will give you problems if you ever want to have low light corals. You need to think this out before rushing out and buying lights only to find out later you made a $500 mistake, IMO

Is it possible to have both low light and high light corals? Just a matter of arranging them closer or further away from the light? I have this same tank... going to tag along with this thread. Thanks!